Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic matrix. These rubber particles are crosslinked to promote elasticity.
In many instances, elastomeric olefinic copolymers (e.g., ethylene-propylene-diene terpolymers) are employed as the rubber component of thermoplastic vulcanizates. For example, U.S. Pat. No. 6,939,918 discloses the manufacture of thermoplastic vulcanizates by employing terpolymers of ethylene, propylene, and diene monomer such as 5-ethylidene-2-norbornene; 1,4-hexadiene; 5-methylene-2-norbornene; 1,6-octadiene; 5-methyl-1,4-hexadiene; 3,7-dimethyl-1,6-octadiene; 1,3-cyclopentadiene; 1,4-cyclohexadiene; dicyclopentadiene; 5-vinyl-2-norbornene, and divinyl benzene. This rubber can be dynamically cured by using any curative capable of crosslinking the elastomeric copolymer including phenolic resins, peroxides, maleimides, and silicon-based curatives.
Thermoplastic vulcanizates that are dynamically vulcanized with peroxide cure systems advantageously are non-hygroscopic, halide-free, lighter in color, thermally stable, and contain less residues. One shortcoming associated with the use of a peroxide cure system is the deleterious impact on the thermoplastic polymers within the thermoplastic vulcanizates. Namely, the peroxide curatives are believed to degrade the thermoplastics (e.g., polypropylene) via chain scission. As a result, thermoplastic vulcanizates that are fully cured by peroxide cure systems may typically be characterized by lower ultimate tensile strength, lower elongation at break, and lower melt strength.
The prior art has attempted to overcome these shortcomings. For example, U.S. Pat. No. 4,985,502 teaches the use of less peroxide curative. Unfortunately, however, the use of a limited amount of peroxide precludes the ability to fully cure the rubber and engineering properties are sacrificed.
Also, U.S. Pat. No. 5,656,693 attempts to alleviate the problem of polypropylene degradation, and yet achieve a full cure of the rubber, by employing a rubber terpolymer that includes vinyl norbornene as a polymeric unit. These rubbers are more efficiently curable with peroxides and therefore the amount of peroxide required to achieve a full cure is reduced, which thereby reduces the impact on the polypropylene.
Inasmuch as the use of peroxide cure systems to dynamically cure—and ideally fully cure—the rubber phase of thermoplastic vulcanizates may offer many advantages, there remains a desire to improve upon the ability to employ a peroxide cure system in the manufacture of thermoplastic vulcanizates.